Instrumented Micropiles Micropiles Instrumented Robert Taylor, - - PowerPoint PPT Presentation

Instrumented Instrumented Micropiles Micropiles

Robert Taylor, P.Eng. International Workshop on Micropiles Schrobenhausen, May 2006

Micropiles Micropiles

• Titan Bars

Titan Bars

• Threaded

Threaded

• Hollow

Hollow

• Drill and install in one operation

Drill and install in one operation

• Through

Through-

• grouted

grouted

• Sizes 30/16 to 73/53 OD/ID

Sizes 30/16 to 73/53 OD/ID

• UTS 220

UTS 220-

• 8000

8000 kN kN

Micropile Micropile Strain Gauge Strain Gauge

• 150mm VW strain gauges

150mm VW strain gauges

• Installed after drilling

Installed after drilling – – no damage no damage

• Uses grout that’s already there

Uses grout that’s already there

• Multiple installations possible

Multiple installations possible

• Protected inside

Protected inside micropile micropile

• Reliable multi

Reliable multi-

• zone load monitoring

zone load monitoring

Sensor Design Sensor Design

• Vibrating wire embedment gauge

Vibrating wire embedment gauge

• Excellent immunity to cable moisture

Excellent immunity to cable moisture

• Cable readily shortened/extended without

Cable readily shortened/extended without effect on results effect on results

• 150 mm length gives averaging of grout

150 mm length gives averaging of grout discontinuities discontinuities

• Compliant carrier centers the gauge on the

Compliant carrier centers the gauge on the neutral axis, facilitates passage through neutral axis, facilitates passage through couplings couplings

Installation Method Installation Method

• Drill and fully grout pile

Drill and fully grout pile

• Finish with high

Finish with high-

• strength cement grout

strength cement grout – – e.g. W/C 0.45 e.g. W/C 0.45

• While grout still fluid, push in VW strain

While grout still fluid, push in VW strain gauge using low gauge using low-

• modulus rods

modulus rods

• Readings as soon as grout sets

Readings as soon as grout sets

• Readings during load application

Readings during load application

Test Program Test Program

• To facilitate interpretation of data from

To facilitate interpretation of data from micropile micropile strain data, controlled load of strain data, controlled load of instrumented bar made instrumented bar made

• Because

Because micropiles micropiles can be applied in tensile can be applied in tensile and compressive directions, both load and compressive directions, both load directions tested directions tested

• Tensile test made with hollow jack

Tensile test made with hollow jack

• Compressive test made on shortened bar

Compressive test made on shortened bar

Specimen Preparation Specimen Preparation

• On production site, test 73 x 53 OD x ID

On production site, test 73 x 53 OD x ID pile drilled 40 cm into soil, grouted, and pile drilled 40 cm into soil, grouted, and gauge inserted gauge inserted

• Following day grout found to have escaped,

Following day grout found to have escaped, exposing gauge exposing gauge

• Bar

Bar regrouted regrouted, tapped to release bubbles , tapped to release bubbles

• Grout cured 25 days in soils laboratory

Grout cured 25 days in soils laboratory

Hydraulic Jack Load Cell

Tensile Test

Test Bar

Compressive Test

1000 2000 3000 4000 5000 6000

• 1000
• 800
• 600
• 400
• 200

200 400 600 800 1000 Load, kN (tension+) microstrain Initial Loading Cycle Titan Bar 73/53 with 6" VWSG-E Strain Gauge cf RST VWA-220-3.937 S/N VC1300 Predicted strain from COSMO FEM model

Results Results

• Proportional response

Proportional response

• Minor non

Minor non-

• linearity on first loading to upper

linearity on first loading to upper values values

• Recovery and reloading response linear,

Recovery and reloading response linear, repeatable; matches prediction of finite repeatable; matches prediction of finite element model element model

Inspection Inspection

• After testing, the specimen bar was

After testing, the specimen bar was sectioned longitudinally sectioned longitudinally

• The sectioning cuts were made by end

The sectioning cuts were made by end milling, flooded in coolant to a residual milling, flooded in coolant to a residual thickness of 1.25 mm thickness of 1.25 mm

• Coolant was excluded using plastic mastic

Coolant was excluded using plastic mastic

• The last 1.25 mm was cut using high

The last 1.25 mm was cut using high-

• speed

speed dry abrasive wheel dry abrasive wheel

Inspection c Inspection continued

• ntinued
• When first side metal was cut through, the

When first side metal was cut through, the micropile micropile sprung open, with the grout sprung open, with the grout remaining tightly retained on one side remaining tightly retained on one side

• The grout had to be pried out, resulting in

The grout had to be pried out, resulting in transverse fracturing in what appeared to be transverse fracturing in what appeared to be intact material intact material

• Artifacts of specimen preparation problems

Artifacts of specimen preparation problems (bubbles, parting at (bubbles, parting at regrouting regrouting surface) had surface) had no apparent effect on performance no apparent effect on performance

Recommendations Recommendations

• It is desirable to install into a fully grouted

It is desirable to install into a fully grouted pile, and to avoid subsequent loss of grout pile, and to avoid subsequent loss of grout – – a positive grout retention system should be a positive grout retention system should be used used

• Caution should be used in inferring load

Caution should be used in inferring load from strains on initial loading from strains on initial loading

• For large peak

For large peak-

• peak strains (>2000

peak strains (>2000 µε µε), ), biasing installation strains and/or biasing installation strains and/or desensitized gauges required desensitized gauges required

Conclusions Conclusions

• After initial loading, subsequent unloading

After initial loading, subsequent unloading and reloading are typically very linear and reloading are typically very linear

• The method appears a robust, practical way

The method appears a robust, practical way to measure load in injection bored to measure load in injection bored micropiles micropiles